📄 obstetrics and gynecology

From Christmas sex to winter intimacy: three decades of birth seasonality, sex ratio dynamics, and fertility change in South Africa, 1994-2024

This study analyzes South African birth data from 1994 to 2024 to reveal a significant shift from Christmas-season to winter conceptions, a stress-induced dip and subsequent spike in the sex ratio linked to the 2021 pandemic mortality peak, and an accelerating decline in annual fertility rates reaching a 30-year low in 2024.

Original authors: Masukume, R., Chimberengwa, P. T., Masukume, G., Liczbinska, G., Grech, V., Mapanga, W.

Published 2026-07-15

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CC BY 4.0

Original authors: Masukume, R., Chimberengwa, P. T., Masukume, G., Liczbinska, G., Grech, V., Mapanga, W.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine South Africa's population as a giant, rhythmic drumbeat. For decades, this drum had a very predictable groove: a massive boom every September. Why? Because nine months earlier, during the festive Christmas and New Year holidays, people were partying, celebrating, and making babies. It was a reliable, holiday-induced rhythm that played out from 1994 all the way to 2014.

But then, the music changed.

The Great Rhythm Shift
Starting in 2015, the September boom began to fade. By 2021, the drumbeat had completely flipped its script. Instead of a September peak, the loudest thump moved to March and April. This suggests that the "baby-making season" shifted from the warm, festive summer holidays to the cooler winter months. The paper doesn't know exactly why this happened—maybe it's because of changing work schedules, how people use their homes, or even the weather—but the data is clear: the old Christmas rhythm is gone, replaced by a new winter intimacy pattern.

The "Stress Test" and the Baby Gender Switch
Here is where things get really interesting, like a sudden glitch in the drum machine. The researchers looked at the sex ratio at birth (the percentage of baby boys versus baby girls). Usually, there are slightly more boys born than girls.

However, between May and July 2021, something strange happened. The number of baby boys dropped so low that for one month (June 2021), there were actually fewer boys than girls. This is a rare event, like a coin landing on its edge.

The paper suggests this wasn't a random fluke. It lines up perfectly with a period of intense national stress: the deadly "Beta wave" of COVID-19 that hit South Africa in early 2021. The researchers propose that the sheer stress and mortality of that time might have subtly altered the biological odds, causing a temporary dip in male births.

But then, the plot thickened again. Just nine months after the country started easing restrictions and getting vaccines in February 2021, the drumbeat swung the other way. In November 2021, the number of baby boys hit its highest point in the entire 31-year study. The authors suggest this might be a "relaxation signal"—a burst of hope and social activity after the worst of the pandemic passed, leading to more conceptions that favored boys.

The Fading Volume
Finally, there's the volume of the music. For a long time, the number of babies born in South Africa was climbing, reaching a peak of 1,112,378 in 2008. But since then, the volume has been turning down.

The paper notes that this decline was already happening, but after 2021, it got much steeper. By 2024, the number of recorded births had dropped to 798,556. This is the lowest level seen in the study. The authors are careful to say this isn't just a temporary pause; the drop is so sharp and sustained that it looks like families are having fewer children overall, not just delaying them.

What the Paper Says (and Doesn't Say)
It's important to know what this study doesn't claim. The researchers are very clear that they cannot prove exactly why these changes happened. They can't say for sure if it was the heat, the economy, or specific family choices. They also don't claim these trends are happening everywhere in the world; they are specific to South Africa's unique experience with the Beta variant and its social changes.

Instead of a solved mystery, the paper offers a set of powerful clues. It shows us that a population's reproductive habits are like a sensitive seismograph, shaking and shifting in response to holidays, stress, pandemics, and societal changes. The rhythm of life in South Africa has changed its tune, moving from a Christmas party beat to a winter whisper, all while the total number of new voices in the chorus is quietly getting smaller.

Technical Summary: Birth Seasonality, Sex Ratio Dynamics, and Fertility Change in South Africa (1994–2024)

Problem Statement
Since South Africa's democratic transition in 1994, the nation has undergone profound social, economic, and public health transformations, including urbanization, electrification, and the impact of the HIV/AIDS epidemic and the COVID-19 pandemic. While seasonal variation in human births is a well-documented demographic indicator of reproductive behavior, the extent to which these societal shifts have altered long-standing patterns of birth seasonality, the sex ratio at birth (SRB), and total fertility rates remains under-analyzed using recent data. Specifically, there is a need to understand how acute population-level stressors, such as the severe Beta-variant wave of COVID-19 in early 2021, may have perturbed reproductive timing and outcomes in a middle-income setting.

Methodology
The study utilized a time-series analysis of national recorded live-birth data from Statistics South Africa, covering the period from January 1994 to December 2024 (372 monthly observations). The analysis focused on three distinct demographic signals:

Birth Seasonality: Monthly birth counts were standardized to average births per day to account for varying month lengths. Months were ranked by intensity within each year. Conception timing was estimated by shifting birth months back approximately nine months. Changes in seasonality patterns were assessed by comparing rank distributions between the periods 1994–2020 and 2021–2024 using the Wilcoxon rank-sum test.
Sex Ratio at Birth (SRB): SRB was calculated as the percentage of male live births. To detect anomalies during the pandemic, an Autoregressive Moving Average (ARMA) model was fitted to pre-pandemic data (January 2015–December 2019). The ARMA(4,3) model, selected via Akaike's Information Criterion (AIC), generated 95% prediction intervals for monthly SRBs from 2020 to 2024. Observed values falling outside these intervals were flagged, and consecutive deviations (May–July 2021) were tested for significance using Fisher's method.
Annual Live Births: A parsimonious linear regression model was fitted to annual birth data from 1994 to 2019 to establish a historical trend. This model was used to generate expected values and 95% confidence intervals for the years 2020–2024 to assess deviations from the historical trajectory.

Key Contributions and Results
The study identified three prominent demographic signals indicating a shift in South Africa's reproductive landscape:

Shift in Birth Seasonality: From 1994 to 2014, September was the consistent peak birth month, corresponding to conceptions during the December–January holiday period. Post-2014, this pattern destabilized. From 2021 onwards, September ceased to be the peak; instead, March (conceptions in June) and April (conceptions in July) emerged as the dominant peak months, with April being the peak in 2024. This indicates a structural shift from "Christmas/New Year" conceptions toward "winter" conceptions.
SRB Perturbation and Inversion: The analysis revealed a rare, sustained depression in the SRB during May, June, and July 2021. The SRB fell below 50% in June 2021 (49.996%), an inversion that occurred within the 3–5 month stress-sensitive window following the January 2021 Beta-wave mortality peak. The combined probability of these three consecutive months falling below the lower 95% prediction bound was highly significant ( $p < 0.001$ ). Conversely, November 2021 recorded the highest monthly SRB of the entire 31-year study period (50.983%), occurring nine months after the easing of restrictions and the arrival of vaccines in February 2021.
Accelerated Fertility Decline: Annual recorded live births peaked in 2008 at 1,112,378 and entered a prolonged decline. While 2020 and 2021 births remained close to historical expectations, the period from 2022 to 2024 saw a sharp acceleration in decline. By 2024, recorded births (798,556) fell significantly below the lower bound of the historical 95% confidence interval, marking the lowest level observed in the study period.

Significance and Claims
The authors posit that these findings serve as complementary indicators of how populations respond to major societal and public health changes, while emphasizing that each signal likely stems from distinct underlying mechanisms:

The shift in seasonality suggests a decoupling of reproductive timing from traditional holiday cycles, potentially driven by urbanization, changing labor markets, and environmental factors, though the specific drivers remain speculative without individual-level data.
The SRB depression in mid-2021 is interpreted as a stress-sensitive demographic signal linked to the severe mortality and societal stress of the Beta-wave, rather than the initial onset of the pandemic in 2020. The subsequent November 2021 SRB spike is hypothesized to reflect a "relaxation signal" following the easing of restrictions, potentially linked to increased partnered sexual activity.
The fertility decline is framed as a continuation of long-term demographic transition trends, but with an acceleration post-2021 that may reflect delayed fertility responses to the severe mortality and economic uncertainty of the 2021 waves, rather than a simple postponement of births.

The paper concludes that while it cannot establish causal pathways due to its ecological nature, the data demonstrates that birth seasonality, SRB, and annual birth counts provide a robust, multi-dimensional view of reproductive dynamics in a middle-income country undergoing rapid change. These signals have practical implications for health system planning, including maternity staffing and immunization resource allocation, particularly given the sharp decline in birth volumes observed in 2024.

Technical Summary: Birth Seasonality, Sex Ratio Dynamics, and Fertility Change in South Africa (1994–2024)

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